| Abstrakt: |
A study of the substituent effects upon the vicinal proton-proton coupling 3JMe to methyl groups has been carried out from data sets of experimental and calculated 3JMe couplings in ethane, denoted 3J0Me, monosubstituted ethanes, denoted 3JxMe, and 1, 1-disubstituted ethabes, denoted 3JxyMe, with substituents of the second (C,N O and F), third (Si, P, S and Cl), fourth (Ge, As, Se and BR) and fifth (Sn, Sb, Te, and I) rows of the periodic table. The calculations of 3JMe couplings were performed at the self-consistent field ab initio level using the 6-31G** basis set. Both the experimental and the calculated 3JxMe couplings of monosubstituted ethanes show a quadratic (or exponential) decay with the relative electronegativity Delta chi x of the substituents in each row. Different decays are found for each row suggesting a periodic behaviour for the effect of the alpha atom upon 3JxMe in addition to the dependence on Delta chi x. The 3JxMe couplings have been analysed using different refined equations formulated from a substituent effect model. A data set of 33 experimental couplings is satisfactorily fitted by two different refined equations including a quadratic dependence on Delta chi x for the effects of the individual substituents and a cross-term on Delta chi x Delta chi y for the effects of interaction between substituents. In the first equa tion, with nine parameters, the periodic trends in the plots of 3JxMe against Delta chi x are considered by using different coefficients for each row in the linear term in Delta chi x. In the second equation, with seven parameters, a unique coefficient is used for the term in Delta chi x and the periodicity is embodied into a linear term in the relative hardness of the substituent Delta etax. [ABSTRACT FROM AUTHOR] |
